The electrical systems in residential, commercial and industrial applications usually include a panelboard for receiving electrical power from a utility source. The power is then routed through protection devices to designated branch circuits supplying one or more loads. These protection devices are typically circuit interrupters such as circuit breakers and fuses which are designed to interrupt the electrical current if predetermined limits of the conductors are surpassed or if other predefined fault conditions are detected on one of the protected lines or branches. Interruption of the circuit reduces the risk of injury or the potential of property damage from a resulting fire.
Circuit breakers are a preferred type of circuit interrupter because a resetting mechanism allows their reuse. Typically, circuit breakers interrupt an electric circuit due to a disconnect or trip condition such as a current overload or ground fault. The current overload condition results when a current exceeds the continuous rating of the breaker for a time interval determined by the trip current. The ground fault trip condition is created by an imbalance of currents flowing between a line conductor and a neutral conductor such as a grounded conductor, something causing a current path to ground, or an arcing fault to ground.
Arcing faults are commonly defined as current through ionized gas between two ends of a broken conductor or at a faulty contact or connector, between two conductors supplying a load, or between a conductor and ground. However, arcing faults may not cause a conventional circuit breaker to trip. Arcing fault current levels may be reduced by branch or load impedance to a level below the trip curve settings of the circuit breaker. In addition, an arcing fault which does not contact a grounded conductor or person will not trip a ground fault protector.
There are many conditions that may cause an arcing fault. For example, corroded, worn or aged wiring, contacts, connectors or insulation, loose connections, wiring damaged by nails or staples through the insulation, and electrical stress caused by repeated overloading, lightning strikes, etc. These faults may damage the conductor insulation and reach an unacceptable temperature. Arcing faults can cause fire if combustible materials are in close proximity.
There are also many conditions that may cause a "false" arcing fault. For example, the occurrence of an arcing fault event in one branch circuit of an electrical distribution system may cause a false arcing fault signal in another branch circuit as a series path is created between the branch circuits through a load center. As a result, circuit interrupters in more than one branch circuit are tripped. Another example is a noisy load such as arc welder, electric drill, etc. producing a high frequency disturbance in the electrical circuit which appears to be an arcing fault.
In general, a "zone" refers to any length of power wiring which is bounded by some definable end of zone device such as a current sensor, a line-to-ground voltage sensor, or a lug between conductors of the same phase. Generally speaking, arc faults are of two types, series arcs and parallel arcs. Series arcs are unintended interruptions in the normal current path such as broken wires, loose terminations or contacts with low contact force. Parallel arcs generally involve conduction through an insulating path between conductors of different voltages. These parallel arcs may be line-to-ground arcs (ground faults) or line-to-line arcs (phase-faults). Generally speaking, the current through a series arc is usually limited by the load impedance while the current through a parallel arc is controlled by the line impedance and the arc voltage.
Prior art arc detection systems leave room for improvement in a number of areas. Some prior art series arc detection devices have relied on noise detection in the load current to detect an arc. Generally speaking, the noise signatures of arcs are broadband current fluctuations which vary in amplitude and frequency content depending on the load type. Many loads also broadcast electrical noise due to electronic power switching components, brush-type motors or other generally "noisy" loads. Some prior art devices have difficulty in distinguishing load noise from arc noise.
Secondly, many arcs begin as a series arc in a failing connection, but are not detected by some prior art detectors until more serious line-to-ground or line-to-line arcs have occurred. That is, some prior art detection methods and devices do not detect a series arc substantially immediately when it is formed.
Thirdly, in line-to-line arc faults, the current is sometimes limited by the arc voltage or line impedance as noted above. Thus, line-to-line faults may not typically be detected by the instantaneous current (magnetic trip) setting of a circuit breaker or the I.sup.2 t time characteristic of a fuse. Moreover, some arcs are sputtering arcs with reduced I.sup.2 t which also slows over current detection by breakers and fuses. Thus, it is difficult with some prior art methods and devices to promptly detect faults of this type.
Fourthly, some prior art methods and devices use algorithms which require some time in which to accomplish signal processing, such as detecting the difference between the present waveform and a reference or time delayed image of the waveform. Such methods may include calculating average load impedances at different frequencies for use in the algorithm. The algorithm must then determine if any observed changes are due to arcs or due to fluctuating load conditions. Since loads may turn on and off and have unpredictable characteristics, arc detection with these methods typically takes some amount of time and is not substantially instantaneous or immediate upon the occurrence of an arc fault. This type of detection can therefore result in nuisance trips, i.e. tripping a circuit breaker due to a fluctuating load condition which is improperly identified as an arc fault, while also potentially missing some arc faults.